Question

mn giúp mình ạ 

Answer 1

1) 1+(-2)+3+(-4)+...+19+(-20)

=[1+(-2)]+[3+(-4)]+...+[19+(-20)]

=(-1)+(-1)+...+(-1)

=(-1).20

=-20

Answer 2

1) Ta có: \(1+\left(-2\right)+3+\left(-4\right)+...+19+\left(-20\right)\)

\(=\left(-1\right)+\left(-1\right)+\left(-1\right)+...+\left(-1\right)\)

\(=-1\cdot10=-10\)

2) Ta có: \(1-2+3-4+...+99-100\)

\(=\left(-1\right)+\left(-1\right)+...+\left(-1\right)\)

\(=-1\cdot50=-50\)